Sputter target and method of using a sputter target

ABSTRACT

A sputtering target used in carrying out a PVD coating process where the sputtering target is sputtered by bombardment with gas atoms and a layer consisting of several metallic elements is deposited onto a substrate, the sputtering target being a plate made of a metal used for building up the layer, and with the other metals used for building up the layer being present at least partially in the form of plugs, which are inserted in holes in the plate, the shape of the free surfaces of the plugs being selected in such a way that the sputtering rate for each metal used in the sputtering process can be set according to the desired composition of the layer being applied.

This is a U.S. National Phase Application Under 35 USC 371 and applicantherewith claims the benefit of priority of PCT/EP01/09119 filed 7 Aug.2001, which was published Under PCT Article 21(2) in English, and GermanApplication No. 100 39 478.7 filed 8 Aug. 2000.

The invention relates to a sputtering target for carrying out a PVDcoating process and a PVD coating process where the sputtering target issputtered by bombardment with gas atoms and/or ions and a layerconsisting of several metallic elements is deposited on the substrate,the sputtering target being a plate made of one of the metals used forbuilding up the said layer, with the remaining metals used for buildingup the layer at least partly in the form of plugs, which are insertedinto holes in the plate.

A target structured in this way is generally referred to as a“mechanical sputtering target”. Before a PVD coating process is carriedout, the sputtering target is fitted to a cathode on the equipment usedfor carrying out the process and is electrically connected to thiscathode. For this reason, the sputtering target possesses the sameelectrical potential as the cathode attached to this equipment.

As a result of bombardment with gas ions, atoms from the plate itselfand the plugs in the plate are removed from the surface of thesputtering target as dust to enter the gas phase. The sputtered-offmetal atoms are separated from the gas phase and deposited on to thesurface of a substrate, such as an indexable insert or a twist drill. Inthis way, the different types of metal atoms are applied to thesubstrate in a certain ratio, thus guaranteeing the stoichiometry of thelayer being applied.

If, for example, a Ti/Al sputtering target is used and a normal TiAlNcoating layer is to be applied to the substrate, the two metals can bepresent in a ratio of 1:1, the overall layer being composed of 25 at %Ti, 25 at % Al and 50 at % N.

With a sputtering target of this type, the plate consists of titanium,while the plugs are of aluminium. In the case of previously knownmechanical sputtering targets, the surface to be treated is flat—i.e.the free surfaces of the plugs lie flush with the surface of the plateitself.

The disadvantage of a known sputtering target of this type is that, atthe beginning of the cathode sputtering process, the ratio (e.g. betweensputtered-off titanium and aluminum atoms) required for building up thedesired layer is not achieved. The sputtering rates do not reach therequired ratio until after a lengthy starting period (often of severalhours' duration) for the sputtering target. This means that the materialthat is already being sputtered off the plate and plugs during therun-in phase is wasted, as it cannot be used in the actual coatingprocess.

In the case—for example—of a mechanical sputtering target with atitanium plate and aluminium plugs, it can be noticed during bombardmentof the target that the sputtering rate for aluminium is considerablyhigher than that of titanium. For this reason, the plugs wear down at afaster rate than the plate itself. The result of this is that a largepart of the plate material remains intact after the plugs have beencompletely worn away.

DE-A-2914618 contains a description of a process for the application ofa sliding or friction surface to a substrate by means of cathodesputtering. The targets used in this process are objects with a circularcross-section, in which the materials used to form the sliding surfaceare combined, the object being made of a matrix material with holescontaining plugs made of other materials.

The article “Oxidation behaviour of nanocrystalline Fe—Ni—Cr—Al alloycoatings” by Z. Liu, W. Gao and Y. He, published in the December 1999issue of “Materials Science and Technology”, Vol. 15, page 1447 ffdescribes a coating process involving Fe—Ni—Cr—Al alloys, which usesmagnetron—sputtering technology. The article cites the use of a targetwith a diameter of 150 mm and a thickness of 6 mm, the eight plugsinserted into the target being of pure aluminium and of 8 mm indiameter, in order to create Fe—Ni—Cr—Al alloy coatings.

DE-A465699 describes a process for the coating of a substrate by meansof cathode sputtering, along with a cathode (target) used for thispurpose. In order to create a regular alloy coating consisting of two ormore materials on the substrate, the target used consists of a metalplate made of one metal, in which pins of a second metal are insertedinto holes. These pins can be used for fixing plates of the secondmaterial to the surface. It is indicated that correct adjustment of theratio of the free surfaces can be used to create the desired coating.

It is the object of the invention to obtain a sputtering target that ismore viable and cost-effective when carrying out a PVD coating processwith cathode sputtering.

This object is achieved, in the case of the item initially referred toas the sputtering target, by selecting the shape of the free surface ofthe plugs in such a way that the sputtering rates for each metalrequired for the desired coating composition are achieved whensputtering the target.

The teaching of the invention consequently involves using the shaping ofthe free surfaces of the plugs to take into account the differentsputtering rates of the metals of the plate and the plugs. Consequently,if the sputtering rate for the plug material is greater than thesputtering rate for the plate material, the free surface of the plugsruns inwards relative to the surface of the plate. In this way, thedrill holes in the plate act as a screen for the gas atoms/ions as theyhit or leave the sputtering target. For this reason, the actualsputtering rate for the plug metal is reduced to such an extent that thesputtering rates required for the desired substrate-coating compositionare reached both for the plate metal and the plug metal.

If, however, the sputtering rate for the plug material is smaller thanthe sputtering rate for the plate material, the free surface of theplugs preferably runs outwardly relative to the surface of the plate.

In comparison to prior sputtering targets, the sputtering target that isthe subject of this invention requires no run-in time—thus resulting ina more cost-effective use of sputtering-target material.

The plate can be made of a single metal. It is however also possible touse a plate made of alloy material. The material used for the plugs canvary from plug to plug. The selection of materials depends exclusivelyon the composition of the layer being applied to the substrate.

Typical combinations of plate and plug materials includetitanium/aluminium, titanium/zirconium and titanium/carbon, the lastmaterial listed being plug material. When carbon is used as a plugmaterial, the plug has a lower sputtering rate than the titanium used tomake the base-plate. In this case, the plug would protrude from thesurface of the plate by the corresponding amount.

It is normally preferable that the actual desired sputtering rate of theplug material is set by selecting the amount by which the plug sinksinto or protrudes out of the holes in the plate.

It is not absolutely vital that the free surface of each plug should lieexactly flush with the surface of the plate. Experiments havenevertheless shown that it is preferable for the free surfaces of theplugs to form a single surface with that of the plate, the free surfacesof the plugs being curved inwards or outwards relative to the plate. Inother words, the free surfaces of the plugs should be linked to thesurface of the plate. The curvature of the free surface of the plugscorresponds, as already described above, to the sputtering-rate ratio ofthe plate material to the plug material.

The holes should preferably be circular in cross-section, while theradius of curvature of the surfaces of the plugs should maintain aratio, with respect to the diameter of the hole, of 1.5:1 to 2.5:1.

In the case of a plug diameter of 15 mm for an aluminium plug in atitanium plate, the best coating conditions are obtained if the radiusof curvature of the free surface of the aluminium plugs has been keptwithin a range of 25 to 35 mm.

If the plug has to be deformed to obtain the desired amount of surfacecurvature, it is preferred that the plug material is softer than thematerial of which the base-plate is made.

The base-plate (which can be made of such materials as titanium, chrome,stainless steel, vanadium, nickel, zirconium, hafnium, tantalum orcarbon) is attached—as known in the art—to a cooling plate, which isnormally made of copper.

The cost-effective use of the sputtering target is also increased bymaking the plugs protrude from the main plate and into holes in thecooling plate (in cases where the plug material wears down faster thanthe plate material). In the case of a titanium plate with aluminiumplugs, the plate can be 5 mm thick, while the plugs are 7 mm long, thusleaving 2 mm of aluminium plug free to protrude into the cooling plate.

After using a sputtering target of this type to apply layers tosubstrates, an optimum amount of titanium is consumed from the plate,despite the higher sputtering rate of aluminium—with 1 mm (for example)of aluminium plug still remaining in the cooling plate. It is notpossible to consume all the titanium in the plate, as a surroundingtrough is formed, during the coating process, between the central areaof the sputtering target and its edge (magnetron target), which is dueto the sputtering process in the sputtering target. Once the depth ofthe trough coincides with the thickness of the base-plate, thesputtering target is no longer usable.

Special attention is drawn to the fact that the method of making theplugs protrude into the cooling plate supposes advantages in itself,quite apart from the items described above with respect to theinvention. For this reason, the configuration of a sputtering target ofthe type described initially (with plugs protruding into the coolingplate) is regarded as a separate solution of the object of thisinvention.

The protrusion of the plugs into the cooling plate offers the additionaladvantage of more effective cooling of the plug material. In particular,this layout also prevents plate material getting between the coolingplate and plugs while the cathode sputtering process is being carriedout. This is of special advantage if the plate material possesses lowheat conductivity (as with titanium, for example).

Different configurations of the invention are further-explained belowwith the aid of drawings. Items illustrated:

FIG. 1 Top view of a sputtering target

FIG. 2 Partial cross-sectional view of a sputtering target with plugs(which have a curved free surface) protruding into a cooling plate.

A typical sputtering target consists of a titanium base-plate, withholes for pressing in the aluminium plugs. The sputtering target isabout 90 mm wide and can be between 100 mm and 800 mm long. Otherdimensions are available as required.

The number of aluminium plugs used depends on the proportion ofaluminium required in the chemical composition of the coating to be usedfor treating the substrate. The proportion of aluminium in thesputtering target can be between 1 and 50 at %.

The plugs typically measure 15 mm in diameter, while base-platethickness can range from 5 mm to 8 mm.

FIG. 1 shows a Ti/Al sputtering target with titanium as base-platematerial and plugs made of aluminium. The aluminium plugs (1) are evenlydistributed across the surface of the titanium base-plate (2).

FIG. 2 shows the section of the sputtering target that is marked with an“X” in FIG. 1. As the cross-sectional view shows, the aluminium plugs(1) have a concave free surface relative to the base-plate (2). The freesurface of the plugs (1) lies flush with the surface of the base-plate(2). The plugs (1) protrude into holes in the cooling plate (3). Theamount of protrusion ensures optimum use of the sputtering target duringthe cathode sputtering process—despite the different sputtering rates oftitanium and aluminium. The purpose of the cooling plate (3), which ismade of copper, is to dissipate the heat produced—during the sputteringof the sputtering target—on the side of the target that is beingsubjected to the sputtering process.

The skilled person selects a concave or convex curvature for the plugsused in each combination of materials for the base-plate (2) and plugs(1), in order to obtain the desired sputtering rate.

In the example of a Ti/Al sputtering target, the aluminium plugsprotrude by at least 1.5 mm into the cooling plate (3), which is between3 mm and 6 mm thick. This shape of the inserted plugs is selected herebecause—with regular bombardment of gas atoms and/or ions—the plugmaterial (in this case: aluminium) possesses a high sputtering rate thanthe plate material (in this case: titanium). The sunken shape of thesurface means that fewer gas atoms and/or ions actually hit the plugmaterial, with the ring-shaped section of surrounding plate-materialacting as a screen.

The skilled person is familiar with the sputtering rates of the variousmaterials used. With these rates in hand, the required shape of thesurface for each combination of plate and plug materials can bedetermined.

1. Sputtering target for carrying out a PVD coating process where thesputtering target is sputtered by bombardment with gas atoms and/or ionssuch that a layer including metallic elements is deposited on asubstrate, the sputtering target comprising a plate comprising aplurality of holes, said plate being formed of a material having asputtering rate and including at least one of the metallic elements fordepositing on the substrate for forming the layer, and a plurality ofplugs, each said plug positioned in a respective one of said holes insaid plate, said plugs being formed of a material including at least onemetal for forming the layer, said material of said plugs having asputtering rate that is different than said sputtering rate of saidmaterial of said plate, each said plug having a free surface that formsa single surface with a surface of said plate, wherein said free surfaceof each said plug is curved outwardly relative to said surface of saidplate when the sputtering rate of the plate is greater than thesputtering rate of the plugs and curved inwardly relative to the surfaceof said plate when the sputtering rate of the plate is less than thesputtering rate of the plugs.
 2. Sputtering target according to claim 1,wherein said holes are of circular cross-section and the radius ofcurvature of the free surfaces of the plugs relative to the holediameter lies within a ratio of 1.5:1 to 2.5:1.
 3. Sputtering targetaccording to claim 2, wherein the plate is made of titanium and theplugs are made of aluminum.
 4. Sputtering target according to claim 3,wherein the hole diameter is 15 mm and the radius of curvature of thefree surfaces of the aluminum plugs lies within a range of 25 mm to 35mm, the free surfaces of the plugs being curved inwards relative to theplate.
 5. Sputtering target according to claim 1, wherein the plugs inthe plate protrude out from said holes and into the cooling plate so asto attach the plate to said cooling plate.
 6. Sputtering target forcarrying out a PVD coating process where the sputtering target issputtered by bombardment with gas atoms and/or ions such that a layerincluding metallic elements is deposited on a substrate, the sputteringtarget comprising a first plate comprising a plurality of holes, saidfirst plate being formed of a material including at least one of themetallic elements for depositing on the substrate for forming the layer,and a plurality of plugs, each said plug positioned in a respective oneof said holes in said first plate and protruding from said holes andinto a cooling plate such that said cooling plate is attached to saidfirst plate by said plugs, said plugs being formed of a materialincluding at least one metal for forming the layer.
 7. Sputtering targetaccording to claim 6, wherein the plugs in the first plate protrude byat least 1.5 mm into the holes in the cooling plate.
 8. A method of PVDcoating, said method comprising the steps of: bombarding a sputteringtarget with gas atoms and/or ions; depositing a layer including severalmetallic elements onto a substrate, said sputtering target comprising aplate comprising a plurality of holes and being formed of a materialincluding at least one of the metallic elements for depositing on thesubstrate for forming the layer, and a plurality of plugs, each saidplug being positioned in a respective one of said holes in said plate;choosing the shape of a free surface of said plugs according to thesputtering rates of the plug and plate material such that the freesurface of each plug is curved inwardly relative to the surface of theplate when the sputtering rate of the plug material is higher than thesputtering rate of the plate material, and the free surface of each plugis curved outwardly relative to the surface of the plate when thesputtering rate of the plug material is lower than the sputtering rateof the plate material; and forming a single layer between the freesurface of the plugs and an upper surface of the plate.